1. Field of the Invention
The present invention relates to canisters trapping fuel vapor vaporized in a fuel tank.
2. Description of the Related Art
A gas vehicle such as automobile is provided with a canister as fuel vapor processor for treating fuel vapor (for example, gasoline vapor) vaporized in a fuel tank. The canister has generally a casing shaped as hollow container, and adsorbents housed in the casing. The casing has a fuel introducing port configured to introduce the fuel vapor vaporized in the fuel tank into the canister, an air communicating port configured to introduce ambient air into the canister, and an exhaust port connected to an air intake pipe for an internal combustion engine. The adsorbents are composed of activated carbon or the like, which can adsorb and desorb the fuel vapor. Thus, the fuel vapor vaporized in the fuel tank flows into the casing through the fuel introducing port and adsorbs onto the adsorbents during parking, etc. Then, ambient air is introduced into the casing through the air communicating port such that the fuel vapor is desorbed from the adsorbents and then is introduced into the engine via the exhaust port together with the air.
When a gas containing the fuel vapor is introduced into an adsorption chamber defined in the casing through the fuel introducing port, the fuel vapor is trapped by the adsorbents in the adsorption chamber. Then, the gas substantially consisting of the air is released into the atmosphere through the air communicating port. However, when a large amount of the fuel vapor flows into the canister during refueling, etc., a portion of the fuel vapor may not adsorb onto the adsorbent and may flow through the canister and into the atmosphere.
Japanese Laid-Open Patent Publication No. 2001-323845 discloses a canister having a plate in a chamber filled with adsorbents. The plate has one or more holes at its upper section for preventing the fuel vapor from passing through the canister and from flowing into the atmosphere. The plate increases airflow resistance in the adsorption chamber so that the fuel vapor remains in the adsorption chamber for a longer time. Accordingly, it is able to increase an adsorption efficiency of the fuel vapor by the adsorbent, resulting in decrease in the fuel vapor released into the atmosphere. However, because some of the adsorbents are disposed between the plate and an air communicating port, the fuel vapor desorbing from such adsorbent may flow into the atmosphere. Therefore, there has been a need in the art for an improved canister.